In image forming devices such as an electrophotographic device, an electrostatic recording device and an electrostatic printing device, a method for forming a desired image by forming an electrostatic latent image on a photoconductor and developing the image with a toner, is widely used. This method is applied to a copying machine, a printer, a facsimile machine, a multifunctional printer, etc.
For example, in an electrophotographic device using electrophotography, generally, the surface of its photoconductor comprising a photoconductive material is uniformly charged by various kinds of methods; an electrostatic latent image is formed on the photoconductor; the electrostatic latent image is developed using toner; a toner image thus obtained is transferred to a recording material such as a paper sheet; and then the toner image is fixed by heating, etc., thereby obtaining a copy.
As the toner used in image forming devices, a toner comprising colored resin particles (toner particles) is generally used, in which an external additive such as inorganic or organic fine particles having a smaller particle diameter than the toner particles, is attached to the surface of the toner particles in order to enhance toner functions such as charge stability and flowability, and obtain a desired printing performance.
However, since the charge amount of the toner is easily influenced by humidity change, there is a problem in that a change in the charge amount of the toner occurs under a high temperature and high humidity environment and causes fogging. Therefore, there is a demand for such a toner that its charge amount is less likely to decrease and is stabilized.
Patent Document 1 discloses a polymerized toner to which, as an external additive, an electroconductive metal oxide and two types of silica fine particles having different particle diameters and being hydrophobized with aminosilane and/or silicone oil are attached. However, when this toner is used in a negative charge development process, there is a problem of fogging.